Testing oscillator



Get. 1, 1935. F FAUSETT 2,016,084

TESTING OSCILLATOR Filed Feb. 5, 1932 2 Sheets-Sheet l *5 f-ll IN VEN TOR ATTORNEYS F. FAUSETT Oct. 1, 1935.

TESTING OSCILLATOR 1932 2 Sheets-Sheet 2 Filed Feb. 5,

cum- O02 com: 33 02 can on: oom onu- O02 cam- 03. co cum 0N omq emu 2.3 82 32 0% saw 0:3 002 Q02 c3 co cmm can 02. 2; 2% 2w can mm 4 0 0 0 4 v con. 0@ 2 ON: Own: 9N2 com 2; 3m 03 .5. 3G 9e .5 2m 8w own a m can omow com Sr 23 onw m m 0% new cow ONQ own 0am a an c3 2L. awe 3w 0: ann c c2. 03 m new cos one an on 03 2h ow- INVENTOR Floyd Fausefi ATTORNEi J Patented Oct. 1, 1935 PATENT OFFICE TESTING OSCILLATOR Floyd Fausett, Greenwood, Miss., assignor to Supreme Instruments Corporation, Greenwood,

Miss.

Application February 5, 1932, Serial No. 591,200

6 Claims.

is sufficiently broad to cover the ordinary broadcast band of tuned radio-frequency receivers as well. as those frequencies dealt with in the intermediate stages of commercial superheterodyne receivers. The broad range of frequencies covered by my oscillator furthermore renders remote its possibilities for becoming obsolete.

It is a further object of my invention to provide an oscillator capable of fine adjustment, with the slightest possibility for variations under the varying conditions that may be imposed upon the same in the course of operations.

It is a further object of my invention to provide a portable testing oscillator capable of being energized by either commercial voltages of alternating or direct current power sources, or by batteries associated with the oscillator, without substantially varying the operating characteristics of the instrument.

It is a further object of my invention to provide a single tube oscillator which is capable of efficient modulation whether it be operated by direct or alternating current energy sources.

Other objects and purposes will appear from the following discussion of the theory of design and the description of my novel oscillator circuit.

The oscillator embraced by this invention is designed and calibrated for universal application for all intermediate and broadcast frequency requirements. This unusual tuning adaptability is accomplished by tuning over a fundamental range of approximately 90 to 250 kilocycles, all higher frequencies being provided by multiples of this fundamental range, which enables the operator tocover a calibrated tuning range from approXi-.

mately 90 to 1590 kilocycles (200 to 3,333 meters), without using complicated and confusing switches. It is, therefore, adaptable to all present commercial intermediate frequencies as well as such frequencies between 90 and 55o kilocycles as may be designed in future radios, thereby greatly lessening the probabilities of obsolescence.

The tuning multiples are obtained by operating the generating vacuum tube with a comparatively high negative grid biasing potential obtained by the voltage drop resulting from the flow of grid current through a resistor in the grid circuit, the grid current being produced by the oscillatory impulses or potentials existing in the grid'circuit. This has the effect of operating the tube on the lower bend of the grid-voltage plate-current characteristic curve of the tube, so that the resultant wave form of the oscillator output signal is composed of a fundamental frequency and several mathematical multiples of the fundamental frequency which is determined by the setting of the tuning dial.

Another advantage found in the use of a grid circuit resistor is that this resistance in series with the internal tube resistance, existing between the grid and filament elements of the tube, constitutes a high resistance which shunts the capacitance and inductance of the tuning circuit, thereby producing the desirable effect of a very low series resistance inthe tuned circuit. This principle has been proven by radio engineers, who have shown that a high resistance shunting a tuned circuit can be be resolved into an equivalent series low resistance by the following formula:

in which R represents the effective series resistance of the tuned circuit and Rsh the resistance which shunts the tuned circuit. From this formula it is also seen that the effective tuned circuit series resistance decreases with increasing frequency (f) or capacity (C). This circuit arrangement has the effect of greatly minimizing the effective resistance of the tuned circuit which is desirable for sharp tuning of the oscillator, that is, the generation of more power at the exact desired frequencies with less power at the undesired frequencies. Thereby it is possible to obtain sharp tuning of the oscillator even with an alternating current power supply.

The multiple tuning arrangement characterizing my oscillator is further advantageous because it provides a desirable apparent tuning broadness at the low frequencies usually employed in the intermediate frequency circuits of superheterodyne radios; that is, a degree of the variable condenser tuning dial represents a more narrow band of frequencies than the degree represents at the higher broadcast frequencies. This characteristic facilitates what is often termed flat topping or staggering adjustments of successive intermediate stages of superheterodyne radios for minimizing the suppression of side band frequencies with resultant tone distortion. This tuning arrangement also permits a certain degree of choice in signal strength and tuning sharpness, as the fundamental and lower frequency signals are somewhat stronger and broader than the higher harmonics which are somewhat weaker and 55" ment l.

more sharply defined. This results in the desired selectivity at all broadcast frequencies.

A very important advantage obtained by the use of the grid resistor is provided by the resultant high input and output impedances of the oscillator tube, so that the internal resistance of the batteries, or other power supply systems, is negligible in comparison with the effective plate resistance of the oscillator tube. This method of operation provides a very stable oscillatory circuit, and the calibration of the oscillator is, therefore, affected very little by changing tubes or batteries, or both, under normal service conditions.

In addition to minimizing frequency variations corresponding to diminishing battery potentials or other power supply variations, the grid biasing resistor already described provides effective protection to the oscillator circuits against grid-toplate short circuits within the oscillator tube.

Such tube element displacements are possible in a portable oscillator, and without this protection, the plate battery could be quickly depleted, or

some part of the circuit could be damaged. if the oscillator is connected to an external power supply system when such element displacements occur.

Similar multiple tuning to that described above could be obtained by the usual practice of operating the oscillator tube with no grid biasing potential, so that the operating point would be on the upper instead of the lower bend of the gridvoltage plate-current characteristic curve, but this method would cause the oscillator tube to operate with high current value and therefore less plate current economy. As hereinafter explained, an additional tube would be required for modulation, which also lessens the operating economy. Furthermore, the oscillator tube would operate with lower input impedance and lower plate resistance, with resultant broader tuning, and wider frequency variations corresponding to tube variations and power supply potential fluctuations or diminishing battery potentials. Also the protection provide-d by the grid resistor described above would not be present.

The oscillator is designed for energization by any one of three types of power supply sources. Either commercial voltages of alternating current energy or direct current energy, or else potentials derived from batteries which may be carried in the same case can be used. The possible opticnal form of energization of the oscillator assures maximum operating economy.

Other features of my circuit are developed hereinafter in conjunction with the detailed description of a preferred embodiment thereof, as illustrated in the accompanying drawings, in which,

Fig. l is a schematic diagram of theosciillator circuit, and

Fig. 2 is a calibration chart adapted to be used in conjunction with the testing oscillator.

In Fig. 1 is illustrated the oscillating audion tube 5, which may be of the 3G type comprising a heated cathode 2, a grid element 3 and a plate ele- In the grid circuit of this oscillator is disposed a resistor composed of two portions 5 and 6, with a condenser 8 in parallel therewith. A modulation toggle switch 7 is designed to'short circuit the portion 5 of this resistor for purposes explained hereinafter in greater detail. The coupling elements between the input and output circuits of the tube consist of inductances 9 and ID with a variable capacity l l disposed across them. The tuning of the oscillator is performed by the variation of capacity I I. Another inductance elethe oscillator is operated by batteries.

ment l2 cooperates with the inductance elements 9 and if! and constitutes the output circuit which transfers the energy developed by the oscillator to a connected radio circuit for utilization.

My oscillator is capable of being operated by 5 any of three types of energy sources. Such a universal mode of energization is particularly advantageous in a portable testing oscillator since the service-man may accurately perform his tests no matter what the nature of the power supply 1 may be. A plug for connecting commercial voltages of either alternating current or direct current energy is indicated at l5. Although normally such a plug would be designed for energization from a 110 volt line, the same could be connected to lines of higher voltage by the insertion of an appropriate resistor in the line to reduce the same to the value normally encountered. Thus when the oscillator is energized from a 220 volt line, a 2090 ohm, 10 watt resistor is inserted in the ex- 2 ternal power supply circuit. A battery supply [3 which may be carried in the oscillator case or else connected externally through leads 23 and 24 serves to energize the plate, when a more economical source of energy is not available. This 5 battery may be a standard 22 volt B battery.

A 4% volt filament battery i4 is also carried in the oscillator case to energize the filament 2 when Battery l4 may be a standard 4 volt C battery. The switches i9 and 2% which may be in the form of a double-pole double-throw switch are employed to selectively connect the circuits from either 110 volt operation with alternating current or direct current energy on one hand, and for operation by batteries on the other hand.

A series of resistors, namely l6, l1 and I8, are placed in the circuit of the power supply in order to provide suitable potentials for the operating elements of the circuit. Thus resistor I3 reduces the line potential to one suitable to be imposed upon plate element 4. Resistor I! further reduces the potential to the point where the same is approximately 4 volts at its left extremity. Resistor i6 furthermore reduces the potential impressed across the filament to 2 volts which is the characteristic filament voltage of the tube employed. This resistor 16 serves as well to reduce the voltage of the battery l4 to 2 volts, which is impressed across the filament 2. 5

The power supply circuits are suitably filtered by condensers 28 and choke coils 29. The condenser 25 serves to exclude the potentials as well as the low frequencies of the power sources from the high frequency portions of the circuit. The

condenser 21 serves to maintain the cathode circuit at ground potential, G, for radio-frequency energy without short-circuiting the ungrounded side of the power supply system, connected to the socket l5. Choke coil 30 as well as chokes 29 block the high frequency energy from the power circuits.

A control of the output of the oscillator is supplied by a resistor 32 connected across the output coil l2. A potentiometer connection wherein the fixed contact is connected to the ground G at 34 and the variable contact to the antenna contact 33 makes possible a variation of the amount of energy transferred from the oscillating circuit to the circuit being tested. "0

The resistor 5, 6 in the grid circuit serves to bias the grid to such a point that several harmonic fundamental frequencies are generated. The advantages caused by this resistance, imparting a high input impedance, and therefore high output impedance to the tube, to stabilize the operating characteristics of the circuit under varying conditions were fully set forth above. The effect upon the sharpness of the tuning of the oscillatory circuit is also one of the advantageous features thereof.

This resistor is furthermore employed to effect the modulation of the oscillations when the circuit is energized by direct current energy. In the case that alternating current energy is utilized, the high frequency energy will be modulated at an audible frequency in dependence upon the frequency of the alternating current energy. However, when the oscillator is energized by either the battery sources or by a ,direct current line potential of 110 volts this mode of modulation is not available. In order to provide for such a contingency, switch 1 is provided. During alternating current operation, the switch is in its closed position to short circuit portion 6 of the resistor, and for direct current operation, it is in its open position as indicated in Fig. 1. The presence of the additional resistor 6 in the grid Y circuit imparts to the oscillating circuit the charsumed that the energy which is fed back from the plate circuit to the grid circuit, which is positive on one half of the cycle and negative on the other, passes through the condenser 8.

I Thus alternating potentials are imposed upon the 7 grid element of the tube.

The positive potentials, corresponding to the positive half of the cycles above, cause an electric current to flow through the grid circuit between the grid and cathode elements and since this current is unidirectional, it cannot flow through the grid condenser, and must therefore flow through the grid resistor, the value of which determines the time element referred to above. If the resistance value of the grid resistor is comparatively low, its action in passing the grid current will be rapid, whereas if the resistance value is high the action will be slow. It is obvious that the value of the resistance with any given capacitor will determine whether or not its leaking functions will be within the audible range, and if in the audible range, the audible frequency or pitch is determined by the value of the resistance.

For the reasons presented in the preceding paragraph, the resistor in the grid circuit is divided into two portions. With the grid modulation switch 1 open, and all of the grid resistance effective, the leaking action is at a suitable frequency and audio modulation is accomplished; with the switch 1 closed only a part of the resistance is effective, and the resulting"leaking action is so rapid that it is beyond audibility and the oscillator is thereby operated without modulation at audio frequency. Between the two grid resistance values effected by manipulation of the modulation switch lies the boundary between audible and inaudible modulation frequencillator for tests involving beat-note tuning methods which requires unmodulated radio-frequency energy.

The grid leak and condenser modulation pitch or frequency varies slightly with the tuning frequency of the oscillator and is advantageous in that it assists in the identification of the tuning multiples. If desired, the operator can obtain from four to ten tuning multiples within the broadcast band without any variations of the 10 modulation pitch at these frequency positions. For instance, the oscillator can be set at an intermediate frequency of 200 kilocycles, and tuned by a radio at five dial positions; namely, 600, 800, 1000, 1200, and 1400 kilocycles, all of these dialing positions having identically the same modulation pitch.

The fundamental frequency and all of the harmonies of the oscillator are present in the output circuit which is coupled from the output coil 12 to the terminals of the radio receiver be ing tested through the terminal jacks A and G. Any desired frequency is selected by means of a calibrationchart associated with the oscillator, which is illustrated in Fig. 2.

The recommended procedure for the frequency selection consists in setting the oscillator at zero and then moving the oscillator tuning dial from zero to a point which will resonate the oscillator with the receiver at any arbitrary tuning of the receiver. This procedure will cause the harmonics of the oscillator to be approximately 250 kilocycles apart. By noting the oscillator dial setting for the resonant condition obtained by the procedure just described, the operator will be able to follow the horizontal line from the Dial setting on the char-t to the curve, thence downward on a vertical line to the fundamental and harmonic frequency indications, where it will be observed that the receiver must be tuned at one of about five frequencies. Since the operator knows the approximate frequency of his receiver; that is, within 200 or 250 kilocycles, he is able to find on the chart the frequency indication which is nearest this approximate frequency. It will be observed that each vertical line of the calibration chart in Fig. 2 represents five times as many kilocycles as the number of the harmonic being used; that is, each vertical line represents 5 kilocycles on the fundamental range, 10 kilocycles i on the 2nd harmonic, 15 kilocycles on the 3rd harmonic, etc. Variations in the number of kilocycles represented by each vertical line is possible in the drawing of the chart. In the chart used commercially, there are five times as many subdivisions as are shown in the accompanying drawings so that each vertical line represents 1 kilocycle on the fundamental range, 2 kilocycles on the 2nd harmonic, 3 on the 3rd, and so on. In choosing broadcast tuning frequencies it is generally advisable to choose the desired frequency at a dial setting between 40 and 50, where the curve of the calibration chart has a slope of about 45 degrees.

The oscillator is provided with the tapered 5;, output control at 32 for controlling the strength -of the radio-frequency signals applied through the dummy antenna to the radio. The oscillator output is so completely controllable that no leakage can be detected on the higher broadcast 70 harmonics when the oscillator is directly connected to a very sensitive radio. Although the direct transmission of the oscillator output energy to the antenna and ground terminals of the radio receiver by means of a shielded transmission line is preferable,'the arrangement operates satisfactorily when the oscillator energy is radiated to the radio broadcast receiver set situated near -by. This is accomplished by circuit and tuning design and by complete shielding. For broadcast adjustments, tuning of any required degree of sharpness is accomplished by choosing higher multiples of lower fundamental tuning frequencies, with proper settings of the output control. 7

Having described my invention,

What I claim is;

1. A radio testing instrument, in combination with a radio broadcast receiving set having .a

' plurality of differently tuned amplification stages,

consisting of a portable oscillation generator comprising an audion tube containing electron emitting' cathode, grid, and plate elements, means for selectively energizing said elements from an external source of alternating current energy, an external source of direct current energy, or a local battery source, means for biasing said grid element to cause distortion of the wave form of the energy generated by said audion whereby a plurality of alternating current waves bearing a harmonic relationship with respect to each other are formed, and electrical contact means for directly impressing only said energy upon said radio broadcast receiving set.

2. In a radio testing instrument, an oscillation generator comprising an audion tube containing electron .emitting cathode, grid, and plate elements, means for selectively energizing said elements from an external source of alternating current energy, an external source of direct current energy, or a local battery source, an input circuit connected between said cathode and grid elements, an output circuit connected between said cathode and plate elements, a grid biasing resistor disposed in said first mentioned circuit to cause distortion of the wave form of the energy generated by said audion, a condenser connected across said resistor, said condenser-resistor combination serving to impose a modulating tone upon the high frequency energy generated by said-audion when the elements thereof are energized by the two last mentioned sources of direct current energy, means for cutting out a portion of said resistor to eliminate said selfmodulation when the elements of said audion are energized the first-mentioned source of alternating current energy, and means for directly impressing said energy upon a radio receiver.

3. In a radio testing instrument, a multiple frequency oscillation generator comprising an audion tube containing filament, grid, and plate elements, a source of high voltage energy, means for switching said filament in series with said source of energy, a resistor in said supply circuit for reducing the potential of said source at a point in said circuit, connecting means from the plate element to said point, another resistor in said circuit for reducing the potential of said source 'at the filament terminals of said audion, a second source of direct current energy, an auxiliary direct current source for said filament, and means for adjusting said switching means to energize said filament from said auxiliary direct current source through a portion of the last mentioned resistor and to connect said plate element to said second source of direct current energy.

4. A radio testing instrument in combination with a radio broadcast receiving set having a plurality of stages of different frequencies, consisting of a multiple frequency oscillation genera-tor comprising a single ,audion tube containing at least an electron-emitting cathode, a grid and a plate element, means for selectively energizing said audion tube by direct current energy or alternating current energy, means for biasing said grid to cause the production by said tube of a plurality of alternating current waves of high frequency bearing a harmonic relation with respect to each other, and means for transmitting only the output energy of said generator to said radio broadcast receiving set for testing said set.

v5. A radio testing instrument in combination with a radio broadcast receiving set having a plurality of stages of different frequencies, consisting of a multiple frequency oscillation generator comprising a single audion tube containing electron-emitting cathode, grid and plate elements, direct current energy sources for energizing said elements, an input circuit connected between said cathode and grid elements, an output circuit connected between said cathode and plate elements coupled to said input circuit, a grid leak assembly in said input circuit comprising a bias ing resistor of such a valueas'to render the tube operative at the lower bend of itscharacteristic curve, whereby waves of harmonic frequencies in addition to the fundamental high frequency Wave are generated, and a condenser in shunt to said biasing resistor, means for modifying said grid leak assembly to selectively obtain modulated and unmodulated high frequency energy in said output circuit, and a transfercircuit for coupling the composite output energy of said generator to .a radio broadcast receiving set fortesting said set.

6. A radio testing instrument in combination with a radio broadcast receiving set having a plurality of stages of different frequencies, consisting of a multiple frequency oscillation generator comprising a single audion tube containing ,electron-emitting cathode, grid and plate elements,

direct current energy sources for energizing said elements, an input circut connected between said cathode and grid elements, an output circuit connected between said cathode and plate elements coupled to said input circuit, a biasing resistor in said input circuitof such a value as to render the. 

